It is known that the life span and the reliability of a workpiece made of tool steel can be considerably improved when the workpiece is coated with a wear-resistant layer. It has already been described in the German publication "VDI-Z" 124 (1982) No. 18, September (II) on page 693, that borides, carbides and nitrides of transition metals, especially titanium carbide or titanium nitride in a thin layer on tool steel improve considerably the life span and reliability of the workpiece.
A suitable process for the coating is the deposition of a chemically reactive gas mixture by CVD (CVD=chemical vapor deposition). Thereby, gaseous compounds containing the elements constituting the layer to be formed by deposition are caused to react with each other at high temperatures. For the deposition of titanium carbide, for example, gaseous titanium chloride (TiCl.sub.4) is reduced in the presence of methane (CH.sub.4) with hydrogen (H.sub.2) as a reducing agent and carrier gas. For the titanium nitride deposition, nitrogen (N.sub.2) is used instead of methane.
Further, it is known to use ceramic Al.sub.2 O.sub.3 based coating materials as the layer of hard material on hard metals, when very high cutting speeds are required in cutting tools. The advantages of a ceramic coating include especially high hardness and heat resistance, a high compressive strength at elevated temperatures, high thermodynamic stability and high chemical resistance. The deposited aluminum oxide is mostly present in the crystalline structure .alpha.-Al.sub.2 O.sub.3 -corundum. As known, all allotropic changes of Al.sub.2 O.sub.3 which result in intermediate stages during the dehydration of the numerous aluminum hydrates tend at high temperatures towards the basic structure of corundum. Since some of these allotropic changes, among others the theta -Al.sub.2 O.sub.3, are stable up to temperatures as high as 1200.degree. C., sometimes, besides the corundum, it is possible to find these allotropic stages in the deposited layer as well. However, research has shown that between .alpha.- and theta-Al.sub.2 O.sub.3 coatings there are no noticeable differences in the cutting (machining) properties of the metal.
However, the drawback of the CVD-coating remains the heretofore required high coating temperature of approximately 1000.degree. C., which leads to losses in the tenacity of the respective composite body. The efforts to develop low-temperature CVD-processes for basic steel bodies, wherein the crystalline growth in the steel as well as a stabilization of the austenite phase can be avoided have lead to the plasma-activated coating process, wherein the reaction gas is superimposed on a non-balanced plasma in a low-pressure glow discharge. Thereby, the temperature of the electrons is considerably higher than the temperature of the ions and neutrons. Compared to a gas in a state of the thermodynamic balance at the same temperature, due to essentially higher energy of the described non-balanced plasma, chemical reactions become possible for which normally much higher temperatures are required.
Low-pressure plasma can be produced in the following different ways:
by applying a constant direct-current voltage to a workpiece functioning as cathode; PA0 by a high-frequency alternating-current voltage; and p0 by a pulsed direct-current voltage (as a consequence of rectangular pulses).
High-frequency pulses can supply inductive or capacitive energy to the reaction vessel for the deposition of very clean layers in electrotechnology (electronics), e.g. for microchips. Since it works with electrodes which are not directly connected to the substrate, it does not matter whether the material itself is conductive or non-conductive. The disadvantage is that this process is very expensive.
The simple way to produce a low-pressure charge is to have the workpiece to be coated set up as cathode and to use the container or its walls as the anode or ground potential. The temperature of the substrate is thereby a function of the voltage and the current.
A further possibility for the plasma-CVD-coating is offered by the plasma-pulse process. Thereby, the temperature of the substrate is a function of the peak voltage and the peak current, as well as of the duration and frequency of the pulses. Advantageously, the temperature of the substrate can be set independently of the parameters of the low-pressure glow discharge the voltage and the current. However, just like in the case of the aforedescribed processes working with a constant direction current, up to now the plasma-pulse process has been used only on metallic conductive materials, e.g. a titanium nitride or titanium carbide coating.
The coating with ceramic layers (Al.sub.2 O.sub.3) is carried out according to the state of the art for instance by a so-called PVD-process (PVD=physical vapor deposition). As an example of the PVD-process cathodic disintegration is mentioned. Thereby, in a glow discharge the material of the cathode disintegrates under the impact of positive ions upon the cathode surface. It is generally known that a coating with titanium carbide is extremely difficult to achieve by cathodic disintegration, since the carbon disturbs the process. As a result, in ceramic deposition by cathodic disintegration only amorphous phases can be obtained.